Bipolar transistors are commonly used in semiconductor devices, especially for high speed operation and large drive current applications. FIG. 1 depicts a conventional bipolar transistor 100. The transistor 100 is shown with a collector region 104 formed in a substrate 102. The collector region 104 is typically a lightly doped epitaxial layer of one conductivity type. A base region 106 is formed in the collector region 104 and has an opposite conductivity type from the collector region 104. An emitter region 108 is formed within the base region 106 and has the same conductivity type as the collector region 104. A collector contact 110, a base contact 112 and an emitter contact 114 are formed on/in the collector region 104, the base region 106 and the emitter region 108, respectively.
For the conventional bipolar transistor 100, electrons flow (and thus current) from the emitter region 108, through the base region 106 to the collector region 110 during normal operation. When there is no current to the base region 106, the transistor 100 is turned off. However, only a relatively small current to the base region 106 causes the transistor 100 to be turned on. The size of the base current regulates the larger amount of current flowing through the transistor, referred to as the collector current, which flows primarily from the emitter region 108 to the collector region 110. There is an amplification effect of the base current to the collector current. The base current has the effect of modifying the resistance of the base region 106. During operation, both negative and positive currents flow through the base region.
The amplification property of the base current to the collector current is referred to as gain, beta, and transistor Hfe. Numerically, the beta is the result of dividing the collector current by the base current. Typical values of beta for DC current gain are generally on the order of 10–200.
The beta value for a given bipolar transistor is generally a design specific parameter for a particular application. However, a desired beta value can often be at odds with a goal of reducing semiconductor device dimensions. Generally, to increase the number of devices per die, it is desirable to reduce depths of the various active regions of the bipolar device (e.g., collector region 104, base region 106, and emitter region 108), which can be counter to achieving a desired beta value.
In addition to influencing beta values, shrinking bipolar transistor device dimensions can also introduce breakdown voltage problems. A breakdown voltage is a voltage value wherein a “breakdown” of a device occurs that results in a short circuit type condition between two or more regions or portions of a device. This short circuit type condition is generally undesirable and can result in significant damage to a semiconductor device, for example, due to high current densities. Generally, it is desirable to have a relatively large breakdown voltage so as to avoid this breakdown effect.
What are needed are bipolar transistor device(s) and/or methods of fabricating such a device that mitigate breakdown voltages while shrinking device dimensions and yet maintaining desired beta values.